1. Field of the Invention
The present invention is directed to a method for operating a nuclear magnetic resonance tomography apparatus, and in particular to a method for rapidly generating a magnetic resonance image with improved diagnostic utility.
2. Description of the Prior Art
Magnetic resonance imaging devices are known in the art which have a first set of coils for generating a fundamental or static magnetic field in which an examination subject is disposed, a plurality of further coils for respectively generating gradient magnetic fields in which the examination subject is also disposed, and a coil or antenna for irradiating the examination subject with a sequence of radio-frequency (RF) pulses and for acquiring the resulting nuclear magnetic resonance signals from the examination subject. In such devices, after each RF pulse, a first gradient pulse in a first direction is generated as a de-phasing gradient, and another gradient pulse in at least one second direction is then generated as a first phase-coding gradient. Following the first phase-coding gradient, a first signal, occurring during a second gradient pulse inverted relative to the first gradient pulse, is read-out as a first read-out gradient.
So-called fast imaging sequences have recently been developed for minimizing the time required to obtain the necessary signals to construct a magnetic resonance image. A so-called FLASH sequence is described, for example, in "Rapid Images and NMR-Movies", Haase et al, SMRM, Fourth Annual Meeting, Book of Abstracts (1985), page 980. In this sequence, excitation of nuclear magnetic resonance signals with RF pulses is undertaken with flip angles substantially below 90.degree., with the fastest possible repetition rate. The image contrast is defined by the longitudinal relaxation time T.sub.1.
Another sequence, known as FISP, is described in "FISP-A New MRI Sequence," Oppelt et al, Electromedica 54, Vol. 1, 1986. In contrast to the FLASH sequence, the phase memory of the spin system is exploited in the FISP sequence by refocusing the coding gradient to obtain an image which is determined by the ratio of the longitudinal relaxation time to the transverse relaxation time (T.sub.1 /T.sub.2).
A FISP sequence of the type utilized in practice is shown in FIG. 2 herein. To obtain nuclear magnetic resonance, the examination subject is exposed to a sequence of RF pulses. A slice selection gradient G.sub.z1 is generated simultaneously with each RF pulse, so that only the slice of the examination subject defined by its z-coordinate is excited. After such excitation has occurred, a gradient pulse G.sub.z2, having a negative direction in comparison to the slice gradient G.sub.z1, is generated. The dephasing of the spin system caused by the slice selection gradient G.sub.z1 is thus cancelled.
A negative gradient pulse G.sub.y1 and a phase coding gradient G.sub.x1 are generated simultaneously with the gradient pulse G.sub.z2. The spin system is de-phased with the gradient pulse G.sub.y1, and the FID signal following the RF pulse is thus destroyed. The phase coding gradient G.sub.x1 is generated after each RF pulse, and impresses phase information on the spin system. The negative gradient pulse G.sub.y1 is followed by a positive gradient pulse G.sub.y2, which in turn cancels the de-phasing caused by the gradient pulse G.sub.y1, and thus generates a spin echo and thus an interpretable signal S.sub.1. After the decay of this signal, a second phase coding gradient G.sub.x2 is generated, which is inverted compared to the first phase coding gradient G.sub.x1. The de-phasing caused by the first phase coding gradient G.sub.x1 is thus cancelled.
A modified FISP sequence known as CE FAST (shown in FIG. 3 herein) was presented by Gingel et al the SMRW Conference in 1986 in Montreal, and is described in "The Application Of Steadystate Free Precession (SFP) in 2D-FT MR Imaging," Gingel et al, SMRM, 5th Annual Meeting, Book of Abstracts (1986), page 666. Again, RF pulses are generated in rapid succession simultaneously with a slice selection gradient G.sub.z1. Each slice selection gradient G.sub.z1 is preceding by a negative gradient pulse G.sub.z3, which cancels the de-phasing caused by the slice selection gradient G.sub.z1. Further, each RF pulse is preceded by a negative gradient pulse G.sub.y4 and by a phase coding gradient G.sub.x1. Following each RF pulse, a positive gradient pulse G.sub.y3, together with a phase-coding gradient G.sub.x2 which is inverted in comparison to the first phase-coding gradient G.sub.x1, are generated. A half echo signal, which is shaped to form a full echo signal by the action of the gradient pulses G.sub.y4 and G.sub.y3, thus arises, as shown in "Phase and Intensity Anomalies in Fourier Transform NMR," Freeman et al, Journal of Magnetic Resonance, Vol. 4, pages 366-383 (1971).
T.sub.1 /T.sub.2 -weighted images with respectively different information content can be produced with the FISP sequence of FIG. 2, and T.sub.2 -weighted images having respectively different information content can be produced with the CE FAST sequence of FIG. 3.